This Month’s Case Was Selected From Several Investigations Related To Failures That Occur When Applying Coating To The Underside Of Metal Deck Panels (Fig. 1). This Case Focuses On An Adhesion Failure Of Coatings Applied To Previously Painted Deck Panels.
A paint contracting company was hired to apply an epoxy ester dry-fall coating to portions of a metal roof deck ceiling inside a warehouse of roughly 650,000 square feet. Metal wall panels and structural steel were also to be coated. The warehouse was at one time a manufacturing facility but had undergone renovation and been converted to a business center.
As Seen in JPCL, October 2017
The surface preparation requirements in the coating specification called for removal of rust, peeling paint, corrosion and other adhesion inhibitors by scraping, sanding, air-pressure blow-down (dry air washing, as the building owner did not want pressure-washing performed inside the building), wire brushing or liquid remover (chemical stripper). The coating to be applied to the metal roof deck and structural steel surfaces in the ceiling area was “Dry-fall, White.” The contractor prepared its bid for interior painting in September 2014 and submitted an epoxy ester dry-fall primer and finish product for application.
The existing paint system being overcoated was reported to be an alkyd product originally applied during the 1980s. There was no record of any other maintenance painting. Near mid-December 2014, a representative from the coating supplier reported that adhesion testing had been performed and that the existing coating system was intact and would support the new coating system.
The epoxy ester dry-fall product data sheet indicated the following.
Typical applications included interior ceilings, support steel, overhead decking, etc.
Recommended thickness: 2-to-4 mils dry, 4-to-8 mils wet per coat, 1-to-2 coats, volume solids 50 percent.
Ambient conditions of air surface and material temperatures between 50 degrees F to 120 degrees F with the surface temperature a minimum of 5 degrees F over dew point and relative humidity maximum of 85 percent. Falls dry in 10 feet at 77 degrees F, and 50 percent relative humidity.
Roughly seven days after beginning application of the dry-fall coating over the existing coating system, delamination and lifting edges developed in the newly applied dry-fall coating system. The failing areas were re-cleaned and recoated. However, coating delamination continued through February and March 2015, during the application of the epoxy ester dry-fall coating.
The contractor reported that an examination of the substrate did not show any evidence of oils or contaminants that would cause delamination. The surfaces containing failed coating were repaired using localized solvent cleaning and the application of a new coat of the epoxy ester dry-fall. However, delamination of the new epoxy ester coating from the existing alkyd continued, as did alkyd delamination. Delamination was also occurring between the newly applied epoxy ester and alkyd on structural steel beams. In addition, the older alkyd coating system was cracked and delaminating from the gray prime coat on the steel, to which the newly-applied epoxy ester remaining adhered.
The coating manufacturer investigated the problem and reported that there were no foreign contaminants on the failure side of disbonded coating chips. The cause of failure was attributed to a combination of application during poor conditions of temperature and humidity, and over moisture. However, the contractor reported that they used heaters when necessary, and the failures continued after the ambient conditions improved.
The investigator visited the warehouse facility in May to examine the dry-fall coating that had been recently applied in the north portion of the warehouse.
Cracked and peeling coatings were observed on back wall panels, the interior roof deck and structural beams supporting the roof of the warehouse. Coating delamination was associated with cracks that were typically linear along metal deck ridges and at times, along wall panel ridges. Some delamination occurred between the light color roof deck panel primer and the alkyd. This was most common where the exposed substrate contained rust staining and rust spots. Delamination occurring on structural-steel members included newly applied epoxy ester separating from the alkyd and the old alkyd delaminating from the original gray prime coat. When the alkyd delaminated from the structural steel, it commonly had the epoxy ester adhered to the alkyd coating.
Coated wall panels included areas where the epoxy ester was applied to surfaces where the alkyd was no longer present and areas where the alkyd system was clearly present (Fig. 2).
Coating adhesion was assessed in accordance with ASTM D3359, “Measuring Adhesion by Tape Test” (Method A, X-cut) (Fig. 3). The adhesion was rated according to the ASTM scale(5A being best and OA being the worst). The adhesion tests were only performed on surfaces that appeared to have intact coating but at times close to areas where failures had occurred.
Adhesion results varied from 4A to OA, with poorer results on surfaces that had lower film thickness as compared to thicker films. Separation from the gray roof deck primer was more common than intercoat separation between the epoxy ester and alkyd coating layer. Probing with a sharp blade revealed that the epoxy ester was more difficult to remove from the alkyd than the alkyd from the deck and steel primer. The brittleness of the epoxy ester contributed to this observation.
Upon scribing, the epoxy ester would crack along the cut edge. However, it was clear on the structural-steel surfaces that the weak bond was between the alkyd and the gray primer when the system was stressed. Spontaneous peeling and delamination occurred between the epoxy ester and alkyd, but also occurred between the alkyd and gray primer when the epoxy ester was tightly adhered to the alkyd. The roof deck panels have large areas where the epoxy ester was present but not adhered. Tapping the epoxy ester with a pen or pencil clearly indicated disbonded coating layers (Fig. 4).
Coating-film-thickness spot measurements were obtained with an electronic coating thickness gage in accordance with ASTM D 7091, “Standard Practice for Nondestructive Measurement of Dry-Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metal.” The thickness of the coating system varied depending on whether the old alkyd was present and the number of individual layers present. A summary of the measurements from the field are shown in Table 1.
Table 1: Coating Thickness Measured over Different Coating Layers.
Coating samples were removed from representative failing and non-failing areas by use of a sharp blade and chisel. Surface samples were obtained by use of cotton swabs used to clean the surface with alcohol during the site visit. Coating samples were placed in individually labeled plastic bags and the solvent swabs were placed in labeled glass vials for safe transport.
The laboratory investigation consisted of visual and microscopic examination and infrared spectroscopy. The microscopic examination of coating chips produced the data shown in Table 2.
Infrared spectroscopic analysis was performed using a Fourier transform infrared spectrometer. Scrapings of the coating samples were combined with potassium bromide powder and formed into pellets under high pressure. Cotton swabs were removed from the vials and extracted with hexane. Hexane was chosen because it would be a stronger solvent than alcohol, but not strong enough to pull the adhesive from the swab. The solvent was then evaporated under a low-temperature heat lamp and potassium bromide powder was added to the resulting residue to produce pellets as with the paintchip samples.
Table 3 shows the source of Samples 10 and 11. Surfaces of Sample 10, “Roof Delaminated” and Sample 11, “Roof Yellow Alkyd” were examined to further investigate the apparently “clean” delamination of the epoxy ester from the alkyd.
Table 2: Laboratory-Measured Coating-Thickness Data.
Table 3: Results of Infrared Spectroscopic Analysis.
The back surface of Sample 11 was the surface against the roof deck primer (and away from the epoxy ester coating). The front surface of Sample 11 was the surface to which the delaminated epoxy ester coating (Sample 10) was applied. The spectrum of the bottom surface of Sample 11 was consistent with an alkyd resin, as expected. However, the spectrum of the top surface of Sample II was not consistent with any common resin type and most likely indicates degradation of the surface of the alkyd. The spectrum appears to show some of the material from the topcoat may have penetrated the degraded area.
THE FAILURE MECHANISM
The failure mechanism that caused the peeling and delamination of the coating layers was two-fold. The primary cause was related to curing stresses. Aged alkyds continue to cure for years by oxidation. Atmospheric oxygen must migrate through the film to combine with deeper portions of the alkyd film. As this progresses, the film’s internal stresses become greater and the film becomes brittle and subject to cracking with age. Cracking typically occurs cohesively through the film thickness as opposed to splitting horizontally within itself. The curing stresses imparted by additional coats of epoxy ester paint that adhered to the alkyd lead to delamination between the primer and old alkyd coating layers once they exceeded the adhesive strength between them. The epoxy ester dry-fall coating failed when the curing stresses, coupled with a weak bonding between it and the alkyd, were sufficient to disbond the epoxy ester essentially cleanly from the alkyd. This produced the disbonding failure shown in Figure 5.
However, when the epoxy ester bond to the alkyd was strong, they did not separate. Rather, the failure occurred between the alkyd coating and original primer – also an alkyd. Under these conditions the epoxy ester and alkyd layers remained bonded to each other.
Simply stated, these adhesion failures occurred because system curing stresses exceeded adhesive and/or cohesive strength of the coating system. These failures not only demonstrate what can occur when curing stresses are too high, but also demonstrate what can happen when intercoat adhesion is too low, as the epoxy ester bond to the alkyd was. Surface contamination is a frequent cause of coating disbonding. Proper adhesion was not achieved when a weak layer existed on the surface of the existing alkyd layers. The analysis of Samples 10 and 11 revealed some degraded surface materials present on the alkyd surface from which epoxy ester delaminated. The speculation that the epoxy ester delaminations were related to the weather conditions during the February to March period is plausible, but does not explain why the same failure occurred even after the weather improved and reapplications were performed.
Excessive film-build of a new coating layer or multiple coats being applied can have the same impact. Increasing the film thickness increases internal stress of the dry, cured film. The coating-layer-thickness data from the laboratory microscopic examination indicated thick coats of the white epoxy ester coats were applied. This exacerbated the peeling of the alkyd coating from original primer when the epoxy ester-to-alkyd bond was strong.
The degree of cleanliness required for overcoating may be hand- or power-tool cleaning (SSPC-SP2,-SP3, respectively) but for large areas such as the warehouse, “Brush-Off Blast Cleaning” (SSPC-SP 7/NACE No. 4) could be employed.
In this case, it was already determined that the existing yellow alkyd coat, even if considered tightly adhered, failed due to stresses of the overcoating product. Therefore, “Industrial Blast Cleaning” (SSPC-SP 14/NACE No. 8) or “Commercial Blast Cleaning” (SSPC-SP 6/NACE No. 3) would be more appropriate.
The original roof deck and steel primer were quite well adhered and the need to remove them might not appear critical. However, at the very least, the original primer should be etched if allowed to remain. Keep in mind that the original primer was an alkyd, just as old as the old, yellowed alkyd coat. Allowing it to remain would likely result in failure by the same mechanism.
The final recommendation was that the existing system be removed using a relatively soft mineral or other abrasive (walnut shells, corn cobs, glass or plastic beads) to avoid distortion and damage to the metal deck substrate. The recommended new coating system was a direct-to-metal acrylic primer and an acrylic overcoat.
By Richard A. Burgess, PCS, KTA-Tator, Inc.